The present invention is directed to a system and method for diagnosing the characteristics of an acceleration sensor inclusive of the deterioration in characteristic thereof, a failure thereof and so on, and more particularly to a diagnosing system and method suitable for an air bag system of an automobile or the like.
Typical examples of a sensor for detecting the acceleration of an automobile include a capacitance type sensor and a piezoresistive (or strain gage) type sensor. In the acceleration sensors of these types, the state of a mass part displaced in accordance with an acceleration is detected from a change in capacitance or a strain.
As a capacitance type acceleration sensor is known one which is formed by a technique for micromachining of silicon or the like. In this known sensor, a pulse width modulation servo technique is applied, as disclosed by JP-A-1-253657.
The acceleration sensor using the pulse width modulation servo technique includes a movable electrode (or mass part) displaced in accordance with an acceleration and at least one pair of fixed electrodes disposed opposite to the movable electrode. One of the pair of fixed electrodes is applied with a voltage in a pulse train form and the other thereof is applied with an inverted version of the pulse voltage. An electrostatic force enabling a control in position of the movable electrode (or an electrostatic servo control) is exerted between the fixed electrodes and the movable electrode by virtue of the applied voltages. When the movable electrode is displaced from a reference position, the displacement is detected from a change in capacitance between the movable electrode and the fixed electrodes.
The electrostatic force is variably controlled by changing the proportion of the application time of the voltage applied to the fixed electrode per a unit period on the basis of a signal representative of the change of the capacitance so that the movable electrode returns to the reference position or the capacitance takes a reference value. The acceleration is detected on the basis of the average value of the voltage applied to the fixed electrode or the signal representative of the change of the capacitance.
There is also known a capacitance type acceleration sensor in which an electrostatic servo is not applied.
U.S. Pat. No. 4,869,092 has disclosed a device for calibration of an acceleration sensor and, more particularly, a technique for calibrating the acceleration sensor by use of an electrostatic force in order to attain a high-precision calibration.